Growth is intimately linked not only to normal development but also to abnormal conditions such as tumorogenesis and cancer. In spite of its importance we still know relatively little about how growth is regulated at the cellular level, at the level of tissues and organs or at the level of the entire organism. Since growth is normally associated with cell multiplication much effort in understanding mechanisms regulating growth has focused on the mechanisms of cell cycle regulation. Indeed, our knowledge of how cells progress through the cell cycle has increased substantially (Nurse 2000). The preoccupation with cell division control has led to the assumption that growth is regulated by factors that control the cell cycle. Elegant experiments in Drosophila imaginal discs, however, reminded us old findings in yeast, that growth regulates the cell cycle and not vice versa (Nurse 1975). In the Drosophila experiments it was shown that accelerating the cell cycle time in clones of cells did not stimulate net growth as measured by the area occupied by the clone, but produced more but smaller cells occupying the same area as the control clones. Conversely, slowing down the cell cycle by overexpression of the RB homolog RBF generated fewer but larger cells again occupying the same area (Neufeld, de la Cruz et al. 1998). Therefore, understanding of growth regulation during normal and abnormal development requires more than understanding cell cycle control. Understanding how growth is regulated at the cellular and tissue level will also provide novel approaches and targets for cancer therapy. Indeed, inhibitors that block cell growth such as Rapamycin are presently in clinical trial as anti-cancer drugs (Hidalgo and Rowinsky 2000). There is therefore an urgent need for means, which allow the diagnosis and the therapy of hyperproliferative diseases.